A. Field of the Invention
The invention relates to an audio transducer to transduce an electrical audio signal into acoustic sound. This invention furthermore relates to a micro speaker optimized for high acoustic output and located within a small volume of a mobile device, such as a mobile phone, a tablet, a gaming device, a notebook or similar device. As the physical volume within these mobile devices is very limited and as the audio transducer has to fit into the housing of the mobile device together with other modules having rectangular shapes, the micro speaker quite often must be constructed having a rectangular form factor.
B. Background Art
When maximizing the performance of a speaker to output high sound pressure an important parameter is a piston wise movement of the membrane. Asymmetry of the mechanical system of a speaker results in asymmetric movements or tumbling of the membrane. This can reduce the sound pressure output power and may result in severe rubbing and buzzing and even damaging of the mechanical system of the speaker. Prior attempts to solve this problem of a tumbling membrane are based on damping membrane materials. The efficiency of such damping, however, can strongly depend on environmental conditions. The invention described herein provides for damping of a tumbling membrane by electrical means and is therefore in a wide range independent from environmental conditions.
Since common membrane designs cannot prevent the system from tumbling, usage of damping membrane material is the most effective and cheap solution. Membrane material, however, has to fulfil many requirements, including having the following characteristics: 1) stabile, frequency-independent stiffness and damping; 2) robustness against mechanical long term stresses; and 3) low cost and good process ability.
Actual materials are always a compromise when it comes to fulfilment of all these requirements, resulting in more or less distortion in the output sound pressure. The resulting total harmonic distortion (THD) is one method used to rate the performance of membranes.
Overcoming tumbling through electrical means requires a method to detect and/or measure the damping during operation of the speaker. One method of doing so is to include a sensor coil wound over the whole height of the voice coil that drives the membrane. The magnetic flux of the magnet system of the speaker will induce a voltage in both coils depending of the actual position of the coil with respect to the magnet system. In a single coil sensor, the induced voltage caused by the forces of tumbling will cancel out due to the fact that the rotational center tends to be through the center of gravity for the coil. The tumbling of the membrane thus cannot be detected.